1. Field of the Invention
This invention relates to composite spherical polymer particles, which contain a nanoparticulate (ultrafine) pigment enclosed therein and are suited as a raw material for cosmetics. This invention is also concerned with their production process and cosmetics using them.
2. Description of the Related Art
In cosmetics, numerous powdery materials are used in view of their functions. Examples of such powdery materials include inorganic color pigments such as titanium oxide, red iron oxide, yellow iron oxide and black ion oxide, inorganic extender pigments such as kaolin, talc and mica, and tar colorants such as Red No. 202 (FDA Name: D&C Red No. 7; CI#: C.I. 15850), Red No. 226 (FDA Name: D&C Red No. 30; CI#: C.I. 73360) and Yellow No. 4 aluminum lake (FDA Name: FD&C: Yellow No. 5 aluminum lake; CI#: C.I. 19140 aluminum lake), and the like. Further, nanoparticles having an average particle size of 0.1 μm or smaller are also mixed in some powders to be added.
Titanium oxide and zinc oxide have conventionally been used in ultraviolet light screening materials. Nanoparticulate titanium oxide and zinc oxide having an average primary particle size of 0.1 μm or smaller are also equipped with high transparency in addition to ultraviolet light screening effect. Therefore, these nanoparticulate titanium oxide and zinc oxide are useful raw materials for UV protection cosmetics (see JP-A-08-053568 and JP-A-09-208437).
Of red iron oxide and yellow iron oxide, nanoparticles having an average primary particle size of 0.1 μm or smaller are called “transparent iron oxides”, and despite their reduced concealing property, are used as brilliant color pigments of excellent transparency in cosmetics. Cosmetics using these pigments are reduced in concealing property, but give highly-transparent, color sensations and also natural finishes generally preferred by consumers. The above-described pigments are, therefore, suited as raw materials for cosmetics.
However, a nanoparticulate pigment subjected to ultrafine grinding to provide it with enhanced functionality is provided with an increased specific surface area and greater surface energy, and therefore, its primary particles are prone to aggregation. It is, hence, difficult to disperse the nanoparticulate pigment into primary particles. If a nanoparticulate pigment is used in the form of a secondary aggregate as a cosmetic raw material, the resulting cosmetic cannot fully exhibit the function which the nanoparticulate pigment is inherently equipped with. Even if such a nanoparticulate pigment is dispersed to primary particles and is then mixed with another cosmetic raw material, the primary particles of the nanoparticulate pigment tend to aggregate for their high surface energy. As a consequence, a cosmetic with a re-aggregated nanoparticulate pigment contained therein undergoes variations with time, and becomes defective in quality such as variations in hue, deteriorations in use feel, and reductions in storage stability (variations in viscosity, settlement, aggregation, discoloration). In addition, a nanoparticulate pigment the average primary particle size of which is 0.1 μm or smaller is so small that a cosmetic with the nanoparticulate pigment mixed therein is poor in spreadability. Further, a cosmetic containing a needle crystal material such as titanium oxide gives an unpleasant use feel as if pricked at the skin, and moreover, its cleansing properties are poor.
In recent years, there is an increasing worldwide concern about the safety of nanoparticles having an average primary particle size of 0.1 μm or smaller. Led by NEDO (New Energy and Industrial Technology Development Organization), a great deal of work is under way in Japan for the development of “a risk assessment of manufactured nanomaterials”. As a cosmetic raw material, nanoparticles have raised a concern that they may give hazardous effects to human health, for example, that for their ability to penetrate through cytomembranes because of their ultrafine size, they may impair the barrier function of skin and not only may remain on the skin but also may penetrate into the body or that they may form active oxygen of high toxicity due to their catalytic action.
In addition, powdery materials for cosmetics, such as carbon black, silica and tar colorants, are also nanoparticles having an average primary particle size of 0.1 μm or smaller, and for similar reasons, there is a concern about their safety.
As has been described above, nanoparticles the average primary particle size of which is 0.1 μm or smaller are suited as a powdery cosmetic raw material, but are accompanied by drawbacks due to their physical and chemical properties, for example, unstable dispersibility, aggregability, thickening property, large specific surface area, high surface activity, hazardousness, and therefore, are inferior in handling ease. For the quality design of a cosmetic with nanoparticles mixed therein, significantly high technologies are thus required for dispersion stabilization, improved storage stability (minimization of viscosity variations, settlement or aggregation, and discoloration), an improved use feel, and the like.